Ground station to satellite 16 systems include mobile satellite programs where multiple spot beams are engaged to connect a large number of mobile users at L-Band or S-Band frequencies to a hub station or gateway between the ground station and satellite operating at Ku-Band or C-Band frequencies. Multiple spot beams are generated using either a phased array or a reflector antenna with an array feed. Current systems may utilize more than 100 feed elements and may beam form combinations of these to generate up to several hundred beams.
Beam forming is implemented by adjusting the amplitude and phase of each signal path routed to each feed element. Each individual signal path is routed to multiple feeds with relative amplitudes and phases which define each intended beam.
On many satellite programs, beam forming is accomplished by constructing a fixed beam forming network behind the feed array. The ports on one side of this network are routed to the feed elements. Ports on the other side are routed by payload hardware in the feeder link to the ground station.
With a fixed beam former, the beam pattern must be configured prior to launch and cannot easily be adapted while in orbit if coverage requirements change after launch. To provide such an ability, recent mobile satellite programs have employed an onboard digital signal processor (DSP) which performs digital beam forming allowing an entire beam pattern to be re-optimized at any time during the life of the spacecraft. The DSP, however, adds significant weight and power to the payload.
Ground based beam forming provides the same or greater flexibility than digital beam forming onboard the satellite without the weight and power penalty of an on-board DSP. The difficulty with ground based beam forming, however, is that multiple paths (one for each feed element) must be maintained between the beam former located on the ground and each feed element located on the spacecraft with relative phase coherency and sufficient amplitude and phase stability to support accurate beam forming.
Most satellite feeder links requiring multiple paths employ Frequency Division Multiplexing (FDM) whereby each path is transferred between the ground station and the satellite on a separate frequency. While coherency is usually not a requirement, it is difficult to maintain coherency in an FDM feeder because the paths are subject to frequency dispersion in either system elements or in the propagation path. As a result most prior art programs requiring beam forming have implemented either fixed or digital beam forming onboard satellite
The present invention overcomes this problem by utilizing a Code Division Multiplexer (CDM) feeder with a novel architecture using orthogonal and pseudo-random CDM codes which permits ground based beam forming with a significant reduction in payload processor complexity. This use of both orthogonal and pseudo-random CDM codes enables a CDM feeder to have the required coherency and stability to support ground system beam forming.